Human malignant and benign prostatic tissue is obtained from surgery and established in monolayer culture. Extracts are prepared from both the surgical tissues and from the in vitro grown cells and are examined by competition radioimmunoassay for the presence of antigens which react with antiserum prepared against antigenic determinants common to RNA tumor viruses. Extracts demonstrating viral antigenic activity are extensively analyzed by a three-phase, two-dimensional immunoelectrophoresis system designed to size and isolate the reacting proteins. Patient sera are also collected and analyzed for immunologic responses to prostatic tissue extracts in radioimmunoassays and for reactions with isolated interspecies viral antigens. Cytotoxic activites of patient sera against prostate cells cultured in vitro are also determined. Localization of viral-like proteins within prostate cells are determined by peroxidase labeling followed by visualization in an electron microscope.